Permanent magnets of cobalt, samarium, gadolinium alloy

ABSTRACT

A permanent magnet having as the active magnetic component a sintered product consisting essentially of cobalt-samariumgadolinium alloy having a reversible magnetization temperature coefficient in air ranging from -0.030% per *C to +0.015% per *C over a temperature range of about -50*C to +300*C, a saturation magnetization 4 pi Js of at least 5 kilogauss and a coercive force Hc of at least -5,000 oersteds.

United States Patent 1 Benz et a1.

1 Aug. 26, 1975 1 PERMANENT MAGNETS OF COBALT,

SAMARIUM, GADOLINIUM ALLOY [75] Inventors: Mark G. Benz, Burnt Hills; Donald L. Martin, Elnora, both of NY.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Aug. 23, 1973 [2]] Appl. No.: 390,722

{52] US. Cl. 148/3157; 148/101; 148/103; 148/105; 75/152 [51] Int. Cl. H01F H04 [58] Field of Search 148/3157, 101, 103, 105; 75/84, 152

[56] References Cited UNITED STATES PATENTS 3,424,578 1/1969 Strnat et a1. 143/3157 3,682,714 8/1972 Martin 143/3157 3,682,715 8/1972 Martin..... 148/3157 3,682,716 8/1972 Martin... 148/3157 3,684,591 8/1972 Martin 143/3157 OTHER PUBLICATIONS Hesbitt, E. et al., Cast Perm. Mag. of CO Retype, in

.lourn. Appl. Phys, 42, March 1971, pp. 15301532.

Buschow, K. et al., Perm, Mag. Mlls. of Rare Earth Compounds, in Zeits. Fur Ang. Physik, 26, 1969, pp. 157-160.

Velge, W. et al., Perm. Mag. Prop. of Rare Earth C0- balt Compounds, in Trans. I.E.E., 1967, pp. 4550.

Das, D., Influence 0f Sim. Temp. on Mag. Prop. of Sm-Co Magnets, in Trans. 115.15., 1971, pp. 432-435.

Primary Examiner-Walter R. Satterfield Attorney, Agent, or Firm-Jane M. Binkowski; Joseph T. Cohen; Jerome C. Squillaro [57] ABSTRACT A permanent magnet having as the active magnetic component a sintered product consisting essentially of cobalt-samarium-gadolinium alloy having a reversible magnetization temperature coefficient in air ranging from 0.030% per C to +0.015% per "C over a temperature range of about 50C to +300C, a saturation magnetization 411'], of at least 5 kilogauss and a coercive force 11 of at [east -5,000 oersteds.

1 Claim, 4 Drawing Figures RELATIVE OPE/V" C PATENTEI] M182 81975 am u 0F 4 PERMANENT MAGNETS OF COBALT, SAMARIUM, GADOLINIUM ALLOY The present invention relates generally to the permanent magnet art and is more particularly concerned with novel permanent magnets of cobalt, samarium, and gadolinium having useful permanent magnet properties and a reversible magnetization temperature coefficient which is significantly low, zero or positive within a broad tempe ture range.

Within the past few years a new class of materials for making permanent magnets has been developed, based on cobalt and rare-earth elements. The improvement over prior art materials is so great that the cobalt-rareearth magnets stand in a class by themselves. In terms of their resistance to demagnetization the new materials are from to 50 times superior to conventional magnets of the Alnico type, and their magnetic energy is from two to six times greater. Since the more powerful the magnet for a given size is the smaller it can be for a given job, the cobalt-rare-earth magnets have applications for which prior art materials cannot even be considered.

Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjunction with the figures accompanying and forming a part of the specification, in which:

FIG. I is the cobalt-Samarium phase diagram. It is assumed herein, that the phase diagram at 300C, which is the lowest temperature shown in the figure, is substantially the same at room temperatures.

FIG. 2 is the cobaltgadolinium phase diagram.

FIG. 3 is a chart bearing curves which illustrate how the substitution of increasing amounts of gadolinium for samarium affects the properties of the resulting ternary alloy magnet. Specifically, the reversible temperature coefficient was measured for the range 20C to 225C, and the remanent induction 8,. and coercive force H were measured at room temperature, 20C.

FIG. 4 is a chart bearing curves comparing the change in open circuit induction 8,, over a broad temperature range of a cobalt-samarium permanent magnet with a cobalt-gadolinium-samarium magnet prepared in substantially the same manner.

When a permanent magnet material is magnetized, a magnetization value of 4w] gauss is established therein. The shape ofthe magnet or the magnetic circuit impose a demagnetizing field of H oersteds. Together, these properties, 4n] and H equal the flux density B which is also measured in gauss.

Probably the most desirable property ofa permanent magnet is that it provide useful external magnetic energy which is constant. This is important because in practically all applications the permanent magnet must function under a variety of demagnetizing influences. Specifically, changes in magnetization with temperature are of concern to the engineer designing permanent magnet devices. Of particular interest is the reversible loss in magnetization or flux which occurs when a magnet is heated to an elevated temperature. It would be desirable for a permanent magnet to undergo no reversible loss in magnetization or flux in the operating temperature range. For example, wattmeters now generally contain conventional magnets which require highly complex circuitry to compensate for changes in magnetization or flux due to changes in temperature and such circuitry adds significantly to the weight and mass of the product.

As used herein, the reversible loss is the change in saturation magnetization 411'], measured at the elevated temperature compared to that measured at room temperature.

Permanent magnets of cobalt and Samarium, particularly those magnets containing a substantial amount of the Co Sm phase, appear to have the most desirable magnetic properties. However, as illustrated by FIG. 3, the permanent magnet of cobalt and Samarium has a reversible magnetization temperature coefficient of 0.045% per C. Such a loss in magnetization may not appear to be significant but over a range of 100 it undergoes a loss of magnetization of 4.5 percent which is significant for a number of applications such as the aforementioned wattmeters.

Heretofore, permanent magnets having a suitable combination of magnetic properties and reversible magnetization temperature coefficient over a desirable broad temperature range have not been produced. In accordance with the present invention, such a magnet is produced and it consists essentially of a sintered product of a ternary metal alloy of cobalt, gadolinium and samarium of particular composition.

Briefiy stated, the present invention is comprised of a permanent magnet wherein the active magnetic com ponent consists essentially of a sintered product consisting essentially of compacted particulate ternary metal alloy of cobalt and a rare earth component. The rare earth component is present in an amount ranging from I6.6 atomic to 20 atomic 7c based on the ternary metal alloy and consists essentially of samarium and gadolinium with the gadolinium atom fraction ranging from 0.12 to less than 0.5 of the rare earth content of the Co-Sm-Gd alloy. The sintered product consists essentially of at least two phases with one of the phases being a Co;,(Gd Sm alloy phase and the sec 0nd of said phases being a Co-Gd-Sm alloy phase richer in rare earth content than the Co,=,(Gd ,Sm alloy phase. The sintered product has a density of at least 87 percent of theoretical and has pores which are substantially noninterconnecting. The present permanent mag net has a saturation magnetization 411], of at least 5 kilogauss, a coercive force H,. of at least 5,000 oersteds and a reversible magnetization temperature coefficient in air ranging from -().O30% per C to +0.0l5% per C over a temperature range of about 50C to +300C.

As a result of the present invention, permanent magnets can be prepared having a predetermined reversible temperature coefficient.

In the present invention the reversible magnetization temperature coefficient per "C is calculated in accordance with the following equation:

4rr./, 4rr.l;-M c 2" 20 [00 reversible temp. coefficient 71 per C.

Specifically, the sintered product is initially magnetized at room temperature to saturation magnetization, 41rJ,,. The resulting magnet is then heated or cooled to the temperature of interest and the magnetization measured at that temperature of interest, 41r.l,, and then the magnetization is measured at room temperature, 47T.lgl| i As used herein, the coercive force, H,., is the demagnetizing field necessary to reduce the induction, B, to

zerov The present magnets have a wide variety of applications. They are highly useful in wattmeters where their 5 particular properties eliminate the need for means which compensate for reversible changes in magnetization or flux due to temperature change. Also, the present magnet with a positive reversible magnetization temperature coefficient may be particularly preferred since in such instance the magnetization or flux would increase with increasing temperature and could be used as a means to compensate for changes in circuitry due to temperature change. In addition, products utilizing the present magnets can be redesigned to have smaller housings and have significantly lower weight.

The present sintered product consists essentially of a ternary metal alloy of cobalt and a rare earth component composed of gadolinium and samarium which is present in an amount ranging from 16.6 atomic 7r to 20 atomic "/1 based on the ternary metal alloy. Should the rare earth content be less than 16.6 atomic 7t of the ternary metal alloy, the sintered product would consist of a single phase C0,,(GdSm) alloy or phases containing less rare earth with properties significantly inferior to those of the present magnet. Also, should the ternary metal alloy contain rare earth in excess of 20 atomic 70, its properties likewise would be significantly inferior to that of the present magnet.

in addition, to produce the present magnet, the gadolinium atom fraction of the rare earth content of the Co-Sm-Gd metal alloy must range from 0.12 to less than 0.5. The particular amount of gadolinium used will depend upon the desired saturation magnetization level, the reversible change with temperature being sought, and the temperature range of interest. Amounts of gadolinium less than the ().l2 atom fraction of the rare earth content do not produce a significant change in the reversible magnetization temperature coefficient per C and atom fractions of gadolinium of 0.5 or higher of the rare earth content of the metal alloy significantly deteriorate permanent magnet properties, e.g., the resulting magnet would not have a saturation magnetization 411'] of at least 5 kilogauss and a coercive force H,. of at least 5()()() oersteds.

The present sintered product consists essentially of at least two phases with one of said phases being a Co ,(Gd Sm alloy phase and the second of said phases being a Co-Gd-Sm phase richer in rare earth content than the Co,r,(Gd,Sm|.,r) alloy phase.

The present sintered product is produced only in accordance with the methods disclosed in U.S. Pat. Nos. 3,655,464, 3,655,463, 3,695,945, and 3,684,593, all of which by reference are made part of the disclosure of es the present application.

Each of the aforementioned patents discloses a process for preparing novel sintered cobalt-rare earth intermetallic products which can be magnetized to form permanent magnets having improved magnetic proper- 6U ties.

Briefly stated, to prepare the present sintered ternary alloy product in accordance with the disclosure of U.S. Pat. No. 3,655,464 a compact of a particulate mixture of a base Co-R alloy which can be Co-Sm, Co-Gd or Co-Gd-Sm and an additive Co-R alloy which likewise can be Co-Sm, Co-Gd or Co-Gd-Sm is sintered to produce a product consisting essentially of at least two phases with one of the phases being a Co ',(Gd,Sm, alloy phase and the second of the phases being a Co-,(Gd,Smalloy phase. Specifically, the base alloy is one which at sintcring temperatures exists as a solid Co R intermetallic single phase. The additive cobaltrare earth alloy is richer in rare earth metal than the base alloy and at sintering temperature it is at least partly in liquid form. The additive alloy may vary in composition and can be determined from the phase diagram for the particular cobalt-rare earth system or it can be determined empirically.

The base and additive alloys, in particulate form, are each used in an amount to form a mixture which has a cobalt, gadolinium and samarium content substantially corresponding to that required in the final desired sin tered product. The additive alloy should be used in an amount sufficient to promote sintering, and generally, should be used in an amount of at least 0.5 percent by weight of the base-additive alloy mixture. The particulate mixture is compressed into a green body of the desired size and density. Preferably, the particles are magnetically aligned along their easy axis prior to or during compression since the greater their magnetic alignment, the better are the resulting magnetic properties.

The green body is sintered in a substantially inert atmosphere to produce a sintered body of desired density wherein the pores are substantially noninterconnecting, which generally is a sintered body having a density of at least about 87 percent of theoretical. Such non-interconnectivity stabilizes the permanent magnet properties of the product because the interior of the sintered product or magnet is protected against exposure to the ambient atmosphere.

Sintering temperature depends largely on the particular cobalt, samarium, gadolinium composition being sintered and generally ranges from about l,()C to about l,200C with a sintering temperature of l,lOOC usually being particularly satisfactory.

The procedure for forming sintered products dis- 0 closed in us. Pat. No. 3,655,463 is substantially the same as that disclosed in U.S. Pat. No. 3,655,464 except that an additive CoR alloy which is solid at sintering temperature and which is richer in rare earth metal than the base alloy is used.

The procedure for forming the sintered products disclosed in US. Pat. No. 3,695,945 is substantially the same as that disclosed in U.S. Pat. No. 3,655,464 except that a cobalt, gadolinium, samarium alloy of proper composition is initially formed.

ln U.S. Pat. No. 3,684,593 there is disclosed a process for preparing heat aged novel sintered cobalt-rare earth intermetallic products by providing a sintered cobaltrare earth intermetallic product ranging in compo sition from a single solid CO5R phase to that composed of C0,,R phase and a second phase of solid Co-R in an amount of up to about 30 percent by weight of the product and richer in rare earth metal content than said COER, and heat-aging said product at an aging temperature within 400C below the temperature at which it was sintered to precipitate Co-R phase richer in rare earth metal content than said Co;,R in an amount sufficient to increase intrinsic and/or normal coercive force of said product by at least it) percent. This particular single phase product is of a composition at sintering temperature close to the boundary or solution line defining the single solid Co -,R phase on the rare earth rich side. Heataging is carried out in an atmosphere such as argon in which the material is substantially inert. The precipitated Co-R phase is generally present in an amount ranging from about I to l5 percent by weight of the product. In the present invention, the precipinetization 4111, at room temperature along its easy axis of magnetization in a field of 60 kiloersteds. The properties of each resulting magnet were determined and are shown in Table l where Run No. I is a control and tated phase is a Co-Gd-Sm phase richer in rare earth 5 where Run Nos. 3-6 illustrate the present invention. content than the Co t,(Gd,Sm,.,) alloy phase. These In Table l the Reversible Temperature Coefficient heat-aged sintered products are particularly useful in per C was determined over the temperature range the present invention. C to 225C.

TABLE I Saturation Energy Product Reversible Rcmanent Gd Magnetization (BH )max. Temperature Coercive Induction Run Gd Sm Fraction of 4-11], 1 It) gauss Coefficient Force H,. B, No. Atomic I4 Gd Sm Kilogauss oersteds) 9? per "C Kiloersteds Kilogauss 1 l6.7 o 10.9 23.4 .04 9.5 9.7 2 I68 .08 9.7 20.1 .u32 1 .211 9.1) 3 [6.7 .121 as 12.3 .U2B 6.5l) 7.6 4 16.8 .42 7.1 93 +004 5. 6.4 5 17.0 .424 6.7 7.6 +1112 5.3 5,6 (1 I6) .4 7.0 9.1 +.ou7 6.() 6.1

The present sintered product is magnetized along the Table I illustrates how the substitution of increasing easy or C axis of magnetization preferably to full satamounts of gadolinium for samarium affects the propuration magnetization or to at least approach saturaerties of the present resulting magnets. Specifically, tion magnetization, ie within about l0 percent of full Run Nos. 3-6, which illustrate the present invention, saturation magnetization. The present permanent magshow permanent magnets with very low or positive renet has a wide variety of applications and is particularly versible temperature coefficients having a saturation useful in wattmeters, computers, and microwave demagnetization of at least 5 kilogauss, a coercive force vices. of at least 5 kiloersteds, and high energy products.

If desired, the sintered bulk product of the present Some of the properties of Table I are shown graphiinvention can be crushed to a desired particle size and cally in FIG. 3 which shows that a gadolinium fraction bonded to a non-magnetic matrix material to produce of 0.12 is necessary to achieve a reversible magnetizathe present permanent magnet. The non-magnetic mation temperature coefficient of 0.030% per C. trix material may vary widely and may be plastic, rub- Metallographic examination of Run Nos. 3, 4 and 5 ber or metal such as, for example lead, tin, zinc, copper showed them to consist essentially of two phases, e.g. or aluminum. a Co (Gd,Sm, phase and a Co-Gd-Sm phase richer The invention is further illustrated by the following in rare earth metal content than the Co ,(Gd,Sm examples. phase. The samples also contained minor amounts of EXAMPLE I an oxide phase conslstmg of Sm O and 0e 0,. Alloy compositions containing between l6 atomic Z EXAMPLE 2 and 37 atomic 7c Sm and/or Gd were prepared by in- In this example a Co-Gd-Sm permanent magnet illusduction melting and chill casting. Each alloy was retrating the present invention was prepared and its propduced to a fine powder ofa particle size between 5-l0 erties compared to a Co-Sm permanent magnet premicrons by a series of steps which included jaw crushpared in substantially the same manner but containing ing, double disk pulverizing, and milling in a fluid enno gadolinium. ergy mill with nitrogen as the working gas. Several Specifically, to prepare the Co-Gd-Sm permanent Co-Sm and Co-Sm-Gd alloy powders were blended t magnet, three powders having an average particle size gether in the appropriate ratio required to achieve the of about 6 microns were blended together to make the desired final average composition in each blend. Also, final blend which was then magnetically aligned along each blend contained at least one powder which functhe easy axis of magnetization and hydropressed to tioned as the solid or liquid sintering additive as set form a green body. The composition of each powder forth in US. Pat. Nos. 3,655,464 and 3,655,463. and the blend 7i of each was as follows:

For each run in Table I, the powder blend was oriented in a magnetic field of 60 kiloersteds along its C or easy axis of magnetization. A low pressure was appnwder WW in WW plied to the oriented sample in order to retain the align- Blend Co Sm (id ment while the sample was transferred to the high pres- A 42,6 657 (I32 I) nu sure chamber. By application ofa hydrostatic pressure 3 13.3 65.5 0.42 34.1 of approximately 13.6 kbars, the oriented sample was C 58 3m compacted to a relative density of percent. The resulting compact was sintered in an atmosphere of argon at a temperature of approximately 1,] 20C for about The composition of the final blend was 62.7 wt.% Co, one hour to densities ranging between and per- 0.6] wt.% 0 20.7 wt.% Sm, and 16.0 WL'7L Gd. Ascent of theoretical. The sintered compact was then fursuming the oxygen to be combined with equal amounts nace cooled to temperatures ranging from 800C to 1,050C at which temperature it was heat-aged for V2 to 25 hours. Each resulting sintered product was about the same size and was magnetized to saturation magof Sm and Gd, the atomic composition of the alloy was calculated to be: 83.3 A/o, Co, 9.8 A/o Sm and 7.0 A/o Gd. The Gd fraction in the metallic phase was 0.4 l 6 or 4L6 atomic 7( of the total rare earth component.

The green body was sintered in an atmosphere of argon at a temperature of l,l40C for 1 hour and then furnace cooled to 925C at which temperature it was heat-aged for 65 hours and then cooled to room temmagnetized in the same manner.

The resulting Co-Sm permanent magnet had an open circuit induction B,, at room temperature of 8.26 kilogauss. The open circuit induction of the Co-Sm magnet perature. 5 was then measured at temperatures ranging from The resulting sintered body was magnetized at room 80C to +80C as illustrated in FIG. 4. FIG. 4 shows temperature in air along its easy axis of magnetization that the open circuit induction of the Co-Sm permain a magnetizing field of 60,000 oersteds. The magnetic nem ma t decreases i ifi l at tcmpgratures properties of the resulting magnet were measured in air ranging from 0C to +75C. The reversible magnetizaat C and then at 225C and the results are shown in o tion temperature coefficient of the Co-Sm magnet was Table ll. determined to be 0.045% per C.

TABLE ll Magnetic Properties 25C 225C Saturation magnetizationAn'J, 7115 K Gauss 7 ll) K Gauss Rcmancnt induction. B, 635 K Gauss 6.31) K Gauss Coercive force. H, Sb'] K ()crs. -5.lh K ()ersv Energy product 9.3 MG Oers. 8.0 MG Oers.

(BH l, l(l"gauss X oersteds) Table ll illustrates that critical magnetic properties What is claimed is: such as saturation magnetization. remanent induction 1. A permanent magnet with substantially stable perand coercive force of the Co-Gd-Sm permanent magmanent magnet properties and characterized by signifinet are substantially constant over temperatures rang- 25 cantly low or zero reversible magnetization temperaing from 25C to 225C. ture coefficient in air through a wide temperature The open circuit induction 5,, of the Co-Gd-Sm magrange consisting essentially of a sintered product of net was determined to be 6.15 kilogauss at room temcompacted particulate ternary metal alloy consisting perature. The Co-Gd-Sm magnet was then cooled to essentially of cobalt and rare earth component, said 8()C and its open circuit induction at various temperrare earth component ranging in amount from l6.6 atures B, ranging from 80C to +80C was deteratomic 7c to 20 atomic of said ternary metal alloy mined. The results are graphically illustrated in FIG. 4 and consisting essentially of samarium and gadolinium which shows a slight positive slope for the Co-Gd-Sm with the gadolinium atom fraction ranging from 0.l2 to magnet over the range 75C to +75C. The reversible less than 0.5 of said rare earth component of said termagnetization temperature coefficient of the Co-Gdnary alloy, said alloy consisting essentially of two pha- Sm magnet was determined to be +0.0047e per C ses with one of said phases being a Co GdSm alloy which indicates that its magnetization 4w] increases phase and the second of said phases being a CoGdSm with increasing temperature. The Co-Gd-Sm magnet alloy phase richer in rare earth content than said had a density of 923 percent of theoretical and its Co GdSm alloy phase. said sintered product having a pores were substantially non-interconnecting. 4O density of at least 87 percent of theoretical and having As a control, a Co-Sm permanent heat-aged was prepores which are substantially noninterconnecting, and pared in substantially the same manner as the Co-Gdsaid permanent magnet having a reversible magnetiza- Sm permanent magnet. The nominal composition of tion temperature coefficient in air ranging from the final CoSm powder blend was 62.75 wt.% Co and 0.030% per C to +0.0l5% per C over the tempera- 37.25 wt.% Sm. A green body of the Co-Sm blend powture range of-SOC to +300C, a saturation magnetizader was formed in the same manner and sintered at a tion 411'], of at least 5 kilogauss and a coercive force H,. temperature 1.1 15C for l hour, then furnace cooled of at least 5,000 oersteds in said temperature range. to 875C at which temperature it was heataged for V2 hour and then cooled to room temperature. It was then 

1. A PERMANENT MAGNET WITH SUBSTANTIALLY STABLE PERMANENT MAGNET PROPERTIES AND CHARACTERIZED BY SIGNIFICANTLY LOW IN ZERO REVERSIBLE MAGNETIZATION TEMPERATURE COEFFICIENT IN AIR THROUGH A WIDE TEMPERATURE RANGE CONSISTING ESSENTIALLY OF A SINTERED PRODUCT OF COMPACTED PARTICULAR TERNARY METAL ALLOY CONSISTING ESSENTIALLY OF COLBALT AND RARE EARTH COMPONENT, SAID RARE EARTH COMPONENT RANGING IN AMOUNT FORM 16.6 ATOMIC % TO 20 ATOMIC % OF SAID TERNARY METAL ALLOY AND CONSISTING ESSENTIALLY SAMARIUM AND GADOLINIUM WITH THE GADOLINIUM ATOM FRACTION RANGING FROM 0.12 TO LESS THAN 0.5 OF SAID RARE EARTH COMPONENT OF SAID TERNARY ALLOY, SAID ALLOY CONSISTING ESSENTIALLY OF TWO PHASES WITH ONE OF SAID PHASES BEING A CO5GDSM ALLOY PHASE AND THE SECOND OF SAID PHASES BEING A COGDSM ALLOY PHASE RICHER IN RARE EARTH CONTENT THAN SAID CO5GDSM ALLOY PHASE, SAID SINTERED PRODUCT HAVING A DENSITY OF AT LEAST 87 PERCENT OF THEORTICAL AND HAVING PORES WHICH ARE SUBSTANTIALLY NON-INTERCONNECTING, AND SAID PERMANENT MAGNET HAVING A REVERSIBLE MAGNETIZATION TEMPERATURE COEFFICIENT IN AIR RANGING FROM-0.030% PER *C TO +0.015% PER*C OVER THE TEMPERATUER RANGE OF -50*C TO +300*C, A SATURATION MAGNETIZATION 4$JS OF AT LEAST 5 KILOGAUSS AND A COERCIVE FORCE HC OF AT LEAST -5,000 OERSTEDS IN SAID TEMPERATURE RANGE 